The separation of hydrocarbon gas mixtures is a common and energy-intensive process in the petroleum refining, natural gas, and petrochemical industries. These mixtures commonly contain methane and heavier hydrocarbons having up to six carbon atoms, and also may contain low concentrations of non-hydrocarbons such as hydrogen, nitrogen, and carbon dioxide. Such gas mixtures include refinery gas streams, raw natural gas, and offgas streams generated in the conversion of heavier hydrocarbons to lighter products.
These hydrocarbon mixtures often are available at elevated pressures up to 1000 psia or higher. A widely-used process for recovering C2 and heavier hydrocarbons from such mixtures involves low temperature fractionation in which a major portion of the required refrigeration is provided by work expansion of pressurized process streams. The autorefrigeration provided by this work expansion may be supplemented by external closed-cycle refrigeration systems using propane, freon, or other working refrigerants.
One such method for recovering light hydrocarbons from mixtures of methane and light hydrocarbons is described in U.S. Pat. No. 4,854,955 wherein an expander process is utilized in which a pressurized feed gas is cooled and partially condensed by heat exchange with returning cold process streams. A portion of the partially-condensed, two-phase feed is separated into a vapor stream and a liquid stream, the vapor stream is cooled by work expansion, and the expanded stream is introduced as a main feed into a low temperature distillation column. The liquid stream is introduced as another main feed into the distillation column. Refrigeration for reflux of the distillation column is provided by further cooling and condensing of another portion of the partially-condensed, two-phase feed, flashing this further cooled stream, and vaporizing a portion of the flashed liquid in the reflux overhead condenser. Partially vaporized feed from the condenser is introduced into the upper portion of the distillation column, above the locations of the main feed streams. Light overhead gas rich in methane is compressed to provide a light gas product and a bottoms product stream enriched in C2+ hydrocarbons is withdrawn from the column.
A related process is disclosed in U.S. Pat. No. 4,889,545 in which a portion of the distillation column overhead vapor is compressed and condensed at an elevated pressure against the vaporizing flashed two-phase feed in a reflux condenser. The condensed overhead is flashed and returned as reflux to the column, and the partly vaporized feed from the condenser is introduced into the upper portion of the distillation column.
Both of the processes described above introduce a significant amount of vapor into the upper portion or rectification section of the distillation column above the locations of the main feed streams. This high vapor loading vapor can have a detrimental effect on the separation efficiency in the rectification section of the column.
The invention disclosed below offers an improved process for light hydrocarbon separation which reduces the vapor load on the rectification section of the distillation column, thereby allowing column operation at higher pressures, reducing reflux condenser duty, and decreasing total power requirements.
The invention relates to a method for the separation of a pressurized hydrocarbon mixture containing at least one more volatile component and at least one less volatile component. In one embodiment, the method comprises
(a) cooling and partially condensing the hydrocarbon mixture to form a two-phase hydrocarbon mixture, and separating a first portion of the two-phase hydrocarbon mixture into a first hydrocarbon vapor and a first hydrocarbon liquid;
(b) work expanding at least a portion of the first hydrocarbon vapor to provide a cooled, expanded hydrocarbon vapor and introducing the cooled, expanded hydrocarbon vapor into a distillation column at a first column location;
(c) reducing the pressure of the first hydrocarbon liquid to provide a reduced-pressure hydrocarbon liquid and introducing the reduced-pressure hydrocarbon liquid into the distillation column at a second column location; and
(d) withdrawing an overhead vapor enriched in the more volatile component from the distillation column; cooling, partially condensing, and separating the overhead vapor to provide a condensed overhead liquid and an uncondensed vapor overhead, introducing the condensed overhead liquid into the distillation column as reflux, and withdrawing from the bottom of the distillation column a stream enriched in the less volatile component.
The cooling and partial condensing of the overhead vapor in (d) may be effected by
(1) further cooling a second portion of the two-phase hydrocarbon mixture to provide a further cooled hydrocarbon mixture;
(2) reducing the pressure of the further cooled hydrocarbon mixture to provide a reduced-pressure hydrocarbon mixture; and
(3) utilizing the reduced-pressure hydrocarbon mixture to provide by indirect heat exchange the cooling and partial condensing of the overhead vapor.
In addition, cooling and partial condensing of the overhead vapor by indirect heat exchange with the reduced-pressure hydrocarbon mixture in (3) may provide a warmed, two-phase hydrocarbon mixture, the warmed, two-phase hydrocarbon mixture may be separated into a second hydrocarbon liquid and a second hydrocarbon vapor, the second hydrocarbon liquid may be introduced into the distillation column, and the second hydrocarbon vapor may be warmed and introduced into the distillation column at a third column location below the first column location of (b).
The cooling of the second portion of the two-phase hydrocarbon mixture of (1) may be effected in part by indirect heat exchange with the second hydrocarbon vapor to provide a warmed second hydrocarbon vapor. The cooling and partial condensing of the hydrocarbon mixture in (a) may be effected in part by indirect heat exchange with the warmed second hydrocarbon vapor to yield a further warmed second hydrocarbon vapor which is introduced into the distillation column at the third column location which is below the first column location of (b). The third column location may be below the second column location.
In another embodiment, a portion of the first hydrocarbon vapor of (a) may be combined with the second portion of the two-phase hydrocarbon mixture of prior to further cooling.
The cooling of the second portion of the two-phase hydrocarbon mixture of (1) may be effected in part by indirect heat exchange with the uncondensed vapor overhead of (d) to provide a warmed uncondensed vapor overhead. The cooling and partially condensing of the hydrocarbon mixture in (a) may be effected in part by indirect heat exchange with the warmed uncondensed vapor overhead.
The overhead vapor enriched in the more volatile component withdrawn from the distillation column in (d) may be compressed prior to cooling and partially condensing, and the partially-condensed overhead may be reduced in pressure prior to introduction into the distillation column as reflux.
If desired, the second hydrocarbon vapor may be work expanded after warming and prior to introduction into the distillation column.
The pressure of the reduced-pressure hydrocarbon mixture of (2) may be lower than the pressure in the distillation column. The second hydrocarbon liquid may be pumped and pressurized prior to introduction into the distillation column. The second hydrocarbon vapor may be compressed prior to being introduced into the distillation column.
The hydrocarbon mixture may comprise methane and one or more hydrocarbons containing two or more carbon atoms. The hydrocarbon mixture also may contain nitrogen, and the hydrocarbon mixture may be natural gas.
In an alternative embodiment, the invention relates to a method for the separation of a pressurized hydrocarbon mixture containing at least one more volatile component and at least one less volatile component. The method of the alternative embodiment comprises
(a) cooling and partially condensing the hydrocarbon mixture to form a two-phase hydrocarbon mixture, and separating a first portion of the two-phase hydrocarbon mixture into a first hydrocarbon vapor and a first hydrocarbon liquid;
(b) work expanding at least a portion of the first hydrocarbon vapor to provide a cooled, expanded hydrocarbon vapor and introducing the cooled, expanded hydrocarbon vapor into a distillation column at a first column location;
(c) reducing the pressure of the first hydrocarbon liquid to provide a reduced-pressure hydrocarbon liquid and introducing the reduced-pressure hydrocarbon liquid into the distillation column at a second column location; and
(d) withdrawing an overhead vapor enriched in the more volatile component from the distillation column, compressing a portion of the overhead vapor to yield a compressed overhead vapor, cooling the compressed overhead vapor to provide a cooled and at least partially condensed overhead stream, reducing the pressure of the cooled and at least partially condensed overhead stream to provide a reduced-pressure overhead stream, introducing the reduced-pressure overhead stream into the distillation column as reflux, and withdrawing from the bottom of the distillation column a stream enriched in the less volatile component.
The cooling of the compressed overhead vapor in (d) may be effected by
(1) further cooling a second portion of the two-phase hydrocarbon mixture to provide a further cooled hydrocarbon mixture;
(2) reducing the pressure of the further cooled hydrocarbon mixture to provide a reduced-pressure hydrocarbon mixture; and
(3) utilizing the reduced-pressure hydrocarbon mixture to provide by indirect heat exchange the cooling of the compressed overhead vapor.
The cooling of the compressed overhead vapor by indirect heat exchange with the reduced-pressure hydrocarbon mixture in (3) may provide a warmed, two-phase hydrocarbon mixture, the warmed, two-phase hydrocarbon mixture may be separated into a second hydrocarbon liquid and a second hydrocarbon vapor, the second hydrocarbon liquid may be introduced into the distillation column, and the second hydrocarbon vapor may be warmed and introduced into the distillation column at a third column location below the first column location of (b).